The mammalian immune system plays a vital role in protection from disease, but its effectiveness rests on a fragile balance of power. Excessive or inappropriate responses result in autoimmune disease, while a failure to respond results in immunodeficiency. When such conditions occur, therapeutic intervention may be required. However, it is not easy to successfully manipulate such a complex system.
The mature cells of the immune system, T cells, B cells and natural killer cells, continually differentiate from hematopoietic stem cells, through a series of cell divisions. It is believed that after each cell division the developmental potential of the daughter cells is either maintained or further restricted relative to the parent, never expanded. One therefore observes that pluripotential stem cells give rise to multi-lineage committed progenitor cells, which give rise to specific lineages and finally mature cells. The coordinated changes of cellular properties leading to irreversible restriction of lineage commitment may be due to sequential activation or silencing of various genes.
The phenotype of long-lived pluripotential hematopoietic stem cells has been described. However, the identification of intermediate bipotent or oligopotent progenitors has been difficult, since the evaluation of differentiating potential may be perturbed by a possible failure for the cells to read out detectable differentiation to particular lineages, which may be due to failure in reaching suitable microenvironments in vivo, an insufficient expansion for detection in vivo, or the stochastic nature of lineage commitment, at least in vitro.
The use of pluripotential of lineage committed progenitor cells circumvents many of the problems that would arise from the transfer of mature cells. However, such progenitor cells must be separated from other hematopoietic cells. Separation requires identification of the cell and characterization of phenotypic differences that can be utilized in a separation procedure. Cells that are amenable to genetic manipulation are particularly desirable.
Relevant Literature
A number of review articles have been published addressing the phenotype of cells in hematopoietic lineages. Overall development of the hematolymphoid system is discussed in Orkin (1996) Curr. Opin. Genet. Dev. 6:597-602. The role of transcriptional factors in the regulation of hematopoietic differentiation is discussed in Georgopoulos et al. (1997) Annu. Rev. Immunol. 15:155-176; and Singh (1996) Curr. Opin. Immunol. 8:160-165.
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A substantially enriched mammalian hematopoietic cell subpopulation is provided, which is characterized by progenitor cell activity for myeloid lineages, but lacking the potential to differentiate into lymphoid lineages. This population is called the common myeloid progenitor cell (CMP). Methods are provided for the isolation and culture of these subpopulations. The CMP population gives rise to all myeloid lineages, and can give rise to two additional progenitor populations that are exclusively committed to either the erythroid/megakaryocytic (MEP) or myelomonocytic lineages (GMP). Both MEP and GMP can be substantially enriched, isolated and cultured. The three progenitor populations are useful in transplantation, for experimental evaluation, and as a source of lineage and cell specific products, including mRNA species useful in identifying genes specifically expressed in these cells, and as targets for the discovery of factors or molecules that can affect them.